NEW YORK (GenomeWeb News) – Many showerheads in American cities are home to opportunistic pathogens, according to a paper that’s scheduled to appear online this week in the Proceedings of the National Academy of Sciences.

Researchers from the University of Colorado swabbed dozens of showerheads in nine American cities, using ribosomal RNA sequences to catalog the microbes present. While most of the microbes in the showerheads were harmless and fairly predictable, the team was surprised to find a potentially pathogenic species called Mycobacterium avium in several showerheads from the metropolitan areas tested.

As part of an Alfred P. Sloan Foundation-funded project on human-microbe overlap in the environment, the researchers decided to focus on showerheads — a source of microbes that many people are exposed to regularly.

“Shower usage provides a source of repeated exposure to microbes through aerosolization and/or direct contact,” senior author Norman Pace, a molecular, cellular and developmental biology researcher at the University of Colorado, and his co-authors wrote. “The inside of a showerhead is a specific niche that is moist, warm, and dark, and frequently replenished with low-level nutrient resources and seed organisms.”

The researchers swabbed the inside of 45 showerheads in nine cities in Illinois, Colorado, Tennessee, North Dakota, and New York State. Their sampling sites included homes as well as public places. A few sites were sampled numerous times to gauge the stability of microbial communities.

By relying on rRNA sequences, the team could identify the bacteria without first culturing them in the lab — a step that misses the majority of microbes in environmental samples, co-author Laura Baumgartner, a post-doctoral research in Pace’s lab, told GenomeWeb Daily News.

For the current paper, the researchers relied exclusively on Sanger sequencing, though Baumgartner noted that they plan to incorporate Roche 454 sequencing into future studies.

The team generated more than 6,090 16S rRNA sequences, which grouped into operational taxonomic units, or OTUs. They then compared the OTUs with known bacterial species to determine what sorts of microbes were present in the showerheads.

Most of the microbes were innocuous and coincided with bugs in nature, especially those that inhabit soil and water. But others were unexpected.

In particular, several samples contained an opportunistic pathogen called M. avium, which is related to the bugs that cause tuberculosis and leprosy. M. avium can cause pulmonary diseases — usually in individuals with compromised immunity. Symptoms of this infection include weakness, fatigue, and a dry, persistent cough.

Overall, M. avium rRNA sequences represented nearly a third of the sequences found in showerhead biofilms, although the microbe was more common in showerheads from two metropolitan areas: New York City and Denver.

“We were starting to conclude that pathogen levels we detected in the showerheads were pretty boring,” lead author Leah Feazel, a former technician in the Pace lab, said in a statement. “Then we worked up the New York data and saw a lot of M. avium.”

And while M. avium was also detected in municipal water supplies in these areas, it was more than 100 times more concentrated in the showerhead biofilm samples.

“If you are getting a face full of water when you first turn your shower on, that means you are probably getting a particularly high load of Mycobacterium avium, which may not be too healthy,” Pace said in a statement.

Other Mycobacterium turned up as well. For instance, the researchers found one showerhead in Denver that was home to M. gordonae, another possible pathogen. M. gordonae was present on that showerhead over three sampling times, despite attempts to clean the showerhead with bleach.

On the other hand, Baumgartner noted, the researchers detected very few sequences from the Legionnaires disease-causing bug Legionella pneumophila, even in samples taken from hospital settings.

Those involved stress that the findings should not dissuade people from showering. Instead, they suggest that those who are heightened risk of opportunistic pathogen infection — such as immunocompromised individuals — may want to think about changing their showerheads regularly or use a showerhead filter.

The researchers also emphasized that they did not detect M. avium in any showerheads fed by well water. And, Baumgartner explained, biofilms form more easily on plastic than on metal showerheads.